Tomorrow's orchards of almonds, pistachios, and walnuts might
be sprayed with fine mists of a beneficial yeast, Pichia anomala.
Studies led by Agricultural Research Service plant physiologist
Sui-Sheng T. (Sylvia) Hua have shown that this yeast can undermine a
troublesome mold, Aspergillus flavus. The mold is of concern because it
produces aflatoxin, a natural carcinogen.

Federal food safety standards and quality-control procedures at
U.S. packinghouses help ensure that these crunchy, healthful tree nuts
remain safe to eat. Nonetheless, growers and processors have a
continuing interest in new, environmentally friendly ways to combat the
mold.

Hua is one of several scientists at ARS's Western Regional
Research Center in Albany, California, who are investigating new
strategies for thwarting A. flavus.

The idea of developing a practical, affordable way for growers to
use a yeast to fight a mold isn't new. But Hua's
tree-nut-focused investigations of P. anomala may be among the most
extensive of their kind to date.

Her research has included exploring the yeast's talents as a
biocontrol candidate in a series of laboratory tests at Albany and in a
field test at a California pistachio orchard. The orchard study,
documented in a patent issued to Hua in 2009, indicated that the yeast
was responsible for a 96-percent reduction in the number of mold spores.

For ongoing laboratory research, Hua has selected, refined, and
applied several analytical procedures to discover precisely how the
yeast disables the mold. "If we understand the underlying
mechanisms," she says, "wemay be able to use that knowledge to
increase the yeast's effectiveness."

In a collaborative experiment with Albany coinvestigators Bradley
J. Hernlem, a chemical engineer; and Maria T. Brandl, a microbiologist,
the mold was exposed to the yeast and later to several different
compounds that fluoresce red or green when evidence of specific changes
in the mold's cells is detected.

Results of these assays, documented in a peer-reviewed article in
the scientific journal Mycopathologia, suggest that the yeast interfered
with the mold's energy generating ATP (adenosine triphosphate)
system, vital for the mold's survival. The findings also suggest
that the yeast damaged mold cell walls and cell membranes. Walls and
membranes perform the essential role of protecting cell contents.

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The team used a different analytical procedure--quantitative
reverse transcriptase PCR (polymerase chain reaction) assays--to analyze
the activity of certain P. anomala genes in the presence of the mold.
Preliminary findings, which Hua reported at the annual national meeting
of the American Society for Microbiology in 2010, suggest that exposing
the yeast to the mold may have triggered the yeast to turn on genes that
code for production of two enzymes--PaEXG1 and PaEXG2.

Though further studies are needed, Hua says these early, PCR-based
findings point to "gene-controlled mechanisms that may be involved
in the cell wall and cell membrane damage observed in the fluorescence
assays."--By Marcia Wood, ARS.

This research supports the USDA priority of ensuring food safety
and is part of Food Safety, an ARS national program (#108) described at
www.nps.ars.usda.gov.